Countermeasures against opportunistic infections are becoming increasingly important due to the increase in patients and elderly persons having depressed immune function due to the use of chemotherapy and other advanced treatment methods in recent years. As is indicated by the fact that opportunistic infections caused by different low virulent organisms are occurring with increasing frequency, the problem of infections will continue to be present as long as there are underlying diseases causing decreases in patients' resistance. Thus, in a society consisting of a large proportion of elderly persons that is certain to appear in the near future, new measures against infectious diseases, including the problem of resistant organisms, are expected to become an important issue.
In the field of antifungal agents, polyene-based amphotericin B or azole-based fluconazole, itraconazole or voriconazole and the like were developed in the past for the treatment of, for example, deep mycoses. Since existing drugs already available on the market frequently have a similar mechanism, the appearance of azole-resistant organisms is currently becoming a problem.
Although naturally-occurring cyclic hexapeptides in the form of caspofungin and micafungin and the like have recently been developed as 1,3-β-glucan synthase inhibitors employing a new mechanism, since these drugs are only available in the form of injections, they are still not adequate as antifungal agents.
In view of the present circumstances in which existing antifungal agents cannot be said to be adequate, there is a strong desire for the development of a highly safe drug based on a new mechanism.    Patent document 1 discloses an example of the prior art relating to an antifungal agent based on such a new mechanism. Patent document 1 describes pyridine derivatives that demonstrate effects against the onset, progression and prolongation of infections by preventing a pathogen from demonstrating pathogenicity by inhibiting the expression of cell wall surface layer protein by inhibiting the transport process of GPI (glycosyl phosphatidyl inositol) anchored protein into the cell wall, and inhibiting the cell wall assembly together with inhibiting the adherence of fungi to cells. However, the group of compounds disclosed in Patent document 1 has 2-benzylpyridine as a common structure thereof, which is clearly structurally different from the compounds according to the present invention. Moreover, although the group of compounds disclosed in Patent document 1 demonstrates activity in vitro, they have problems in vivo such as being easily metabolized.
However, patent documents 2 to 8 disclose examples of the prior art that the most structurally similar to the pyridine derivative (I) as claimed in the present invention. Patent document 2 describes N-(4-pyridyl)carboxamide derivatives having effects as an agricultural chemical, and particularly as an insecticide, miticide or nematocide. Patent documents 3 to 6 describe 2-aryloxynicotinamide derivatives having inhibitory action on phosphodiesterase 4 (PDE4), while patent document 7 describes 6-(arylamino)nicotinamide derivatives having cannabinoid receptor regulatory action, and patent document 8 describes 6-(aryloxy)nicotinamide derivatives having Na+/Ca2+ exchanger inhibitory action. However, compounds according to the present invention are not described in any of Patent documents 2 to 8, and antifungal action against ordinary fungal species such as Candida, Aspergillus or Cryptococcus species and the like are not disclosed at all in the compounds disclosed in Patent documents 2 to 8.    Patent document 1: the pamphlet of International Publication WO 02/04626    Patent document 2: the specification of U.S. Pat. No. 5,852,042    Patent document 3: the specification of European Patent Publication No. 1229034    Patent document 4: the pamphlet of International Publications WO 02/060875    Patent document 5: the pamphlet of International Publication WO 02/060896    Patent document 6: the pamphlet of International Publication WO 03/068232    Patent document 7 the pamphlet of International Publication WO 2004/029027    Patent document 8 the pamphlet of International Publication WO 2004/000813